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Like-chargeion pairing is commonly observed in protein structuresand plays a significant role in biochemical processes. Density functionalcalculations combined with the conductor-like polarizable continuummodel were employed to study the formation possibilities of doublycharged noncovalently linked complexes of a series of model compoundsand amino acids in the gas phase and in solution. Hydrogen bond interactionswere found to offset the Coulombic repulsion such that cation–cationclusters are minima on the potential energy surfaces and neither counterionsnor solvent molecules are needed to hold them together. In the gasphase the dissociation energies are exothermic, and the separationbarriers span from 1.7 to 15.6 kcal mol–1. Liquid-phasecomputations indicate that the separation enthalpies of the cation–cationcomplexes become endothermic in water and nonpolar solvents with dielectricconstants of ≥7 (i.e., the value for THF). These results revealthat electrostatically defying noncovalent complexes of like-chargedions can overcome their Coulombic repulsion even in low-polarity environments. [ABSTRACT FROM AUTHOR]